Carbonylation processes are well known in the art. Of particular commercial significance are processes for the carbonylation of methanol to make acetic acid and processes for the carbonylation of methyl acetate to make acetic anhydride. See Applied Homogeneous Catalyst With Organometallic Compounds, Cornils et al., Ed. (Bench Edition) (Wylie, Weinheim, Federal Republic of Germany 2000), Chapter 2, Parts 2.1.2 and following, pp. 104-137.
To make acetic acid, one method of choice involves carbonylating methanol in a homogeneous reaction medium wherein rhodium is utilized as a catalyst. This method is sometimes referred to in the art generally as the Monsanto process and was developed in the 1970's. A more preferred carbonylation process is taught in U.S. Pat. No. 5,144,068 to Smith et al. In this so called “low water” process, an alcohol such as methanol is reacted with carbon monoxide in a liquid reaction medium containing a rhodium catalyst stabilized with an iodide salt, especially lithium iodide along with alkyl iodide such as methyl iodide and alkyl acetate such as methyl acetate in specified proportions. With a finite concentration of water in the reaction medium, the product is the carboxylic acid instead of, for example, the anhydride. The reaction system of the '068 patent not only provides an acid product of unusually low water content at unexpectedly favorable rates, but also exhibits unexpectedly high catalyst stability. That is, the catalyst is resistant to catalyst precipitation out of the reaction medium.
Another method of choice for carbonylating methanol involves utilizing a homogeneous iridium catalyst in the reactor. There is disclosed, for example, in U.S. Pat. No. 5,883,295, to Sunley et al. a process for the production of acetic acid comprising carbonylating with carbon monoxide methanol and/or a reactive derivative thereof, in the substantial absence of a metal promoter and/or ionic iodide co-promoter in a carbonylation reactor containing a liquid reaction composition with an iridium carbonylation catalyst, methyl iodide co-catalyst, water, acetic acid, and methyl acetate wherein there is maintained in the liquid reaction composition: (a) water at a concentration of less than 5% by weight; (b) methyl iodide in a concentration of greater than 12% by weight and (c) in the carbonylation reactor a total pressure of less than 50 bar. See also U.S. Pat. No. 5,877,348 to Ditzel et al. and U.S. Pat. No. 5,887,347 also to Ditzel et al.
One drawback of homogeneous systems is the tendency of the catalyst to form volatile species which leads to catalyst loss. See, for example, U.S. Pat. No. 5,942,460 to Garland et al. at Col. 4, lines 3 and following as well as U.S. Pat. No. 5,932,764 to Morris et al. at Col. 3, line 1 and following wherein it is stated:                . . . Preferably to prevent a significant increase in the volatility of the iridium catalyst and/or optional promoter the amount of carbon monoxide in the second liquid composition withdrawn from the second reaction zone should not be reduced too low, typically to maintain at least 20% by volume of the dissolved and/or entrained gases therein . . . .        
Supported catalysts systems have been proposed to avoid excessive catalyst loss through entrainment. For example, U.S. Pat. No. 5,466,874 to Scates et al., U.S. Pat. No. 5,281,359 to Scates et al., U.S. Pat. No. 5,334,755 to Yoneda et al., U.S. Pat. No. 5,364,963 to Minami et al., U.S. Pat. No. 5,155,261 to Marston et al., U.S. Pat. No. 5,892,110 to Ramprasad et al., U.S. Pat. No. 4,127,506 to Gray et al., WIPO Publication WO 98/57918, and WIPO Publication 98-33590. A significant drawback of using supported catalyst systems in carbonylation processes has been the leaching of the catalyst from the polymer; this drawback may be avoided, at least in part, by utilizing the present invention as discussed hereinafter.
Rather than using a supported catalyst, another approach has been to sequester catalyst that is entrained in process streams by using polymeric substrates. Methods of this type are described, for example, in European Patent Specification No. 1 315 693 to Johnson Matthey, and U.S. Pat. No. 6,662,770 to Cheung et al., which describe processes employing resin beds to sequester catalyst from process streams in which the entrained catalyst is present in the parts per million range.
It has been discovered in connection with the present invention, that a convenient and effective method for recovering metal catalyst in carbonylation processes can be provided by first generating a process stream having less than 100 ppb by weight of catalyst metal, and treating the process stream with a polymer having nitrogen-containing heterocyclic repeat units. Because the catalyst is present in low concentrations, moderately sized polymer beds can be used without the need for constant replacement of the polymer. Moreover, the inventive methods and apparatus enable the reclamation and re-use of valuable catalyst which would otherwise be lost to entrainment and/or volatilization.